It was one of the most influential rock albums of all time and arguably the last rock album to drastically change the course of popular music. And come Sept. 24, the album in question, Nirvana’s “Nevermind,” will mark its 20th anniversary with as much fanfare as you’d expect from such a significant piece of pop culture history.

For starters, the album is getting re-released in two versions. There’s a two-CD “deluxe edition” which has the remastered album plus B-sides, live cuts and studio sessions. There’s also a four-CD “super deluxe edition” with all of the above, plus the first official release of the pre-“Nevermind” demos, an alternate mix of the album, a pair of previously unreleased BBC recordings, and a recording of a live 1991 show at Seattle’s Paramount Theater featured on both CD and DVD.

Out in medialand, SPIN magazine put together a free, downloadable tribute album of “Nevermind” cover versions and Alternative Press magazine had 20 bands share their thoughts about the album. Comedy Central’s Jon Stewart is hosting a Q&A session on SiriusXM with surviving band members Dave Grohl and Krist Novoselic Sept. 24 and SiriusXM is launching a temporary all-Nirvana channel, Nevermind Radio, Sept. 23-28.

“Nevermind” has been credited over the years with commercializing grunge and alternative rock, ushering in a more serious era of hard rock, helping to kill off hair metal and establishing Seattle as a musical force (even though Nirvana itself was from Aberdeen, Wash.)

But two decades later, it's still unclear why “Nevermind,” of all albums, became so “contagious,” to quote its lead single, “Smells Like Teen Spirit.” Why didn’t albums by Nirvana’s peers, like Alice in Chains, or predecessors like Husker Du, set the world alight?

Timing is (almost) everything
According to Charles R. Cross, the author of “Heavier Than Heaven,” a biography of Nirvana front man Kurt Cobain, both musical and non-musical factors led to the mega-success of “Nevermind.”

“It’s an incredible album,” Cross said. “It would have been a hit whenever it was released. But at the same time, the timing was right for there to be a superstar act like Nirvana. It came right at the end of the death knell of hair metal and the world was screaming for rock music that would be meaningful again. And the timing for a new generation wanting a voice was also ideal.

“It just so happened that everything came together at the exact right moment when rock needed a revolution,” Cross said.

Bands from Seattle had already landed some albums on the charts and the city’s music scene was poised for a mainstream breakthrough, said Jack Endino, who produced Nirvana’s debut album “Bleach.”

“People were starting to pay attention to the Seattle thing,” Endino said. “People don’t remember that before ‘Nevermind’ came out we already had Soundgarden on the charts and we had (the Top 50 album) “Facelift” by Alice in Chains and “Uncle Anesthesia” by Screaming Trees. People were already beginning to pay attention to what was coming out of Seattle because it was kind of unprecedented for a place like Seattle to suddenly have rock albums on the album charts — not just one band but two, three or four bands.”

What Endino said Seattle needed at the time was “one really big breakthrough album to come through to really get people to focus on it.” And that’s what “Nevermind” did, he said.

“What the ‘Nevermind’ record had was good melodies,” said Endino. “People forget that ‘Smells Like Teen Spirit’ has a very Beatlesque melody. That’s not a trivial melody. That’s a very sort of sophisticated melody to actually hear in a rock ’n’ roll song, especially at that point in time when hip hop was coming along which was not particularly melodic.”

Said rock writer Chuck Eddy: “It had a great chord progression that worked in ‘Louie Louie’ and ‘More Than a Feeling.’ And it had a great video. Not only was (Cobain) a pretty guy, but he also wrote pretty melodies.”

Few at the time predicted such success for the album, Nirvana’s first effort for a major record label, Geffen Records. According to Chad Channing, who drummed on “Bleach,” and worked on some of the early “Nevermind” songs, the album’s explosion on the pop charts wasn't foreseen by either the band or its core audience.

“Playing live, I couldn’t really discern any difference between stuff that we were playing from the ‘Bleach’ record and then the new stuff,” Channing said. “They seemed to like it all.”

In the studio, Channing said, “we didn’t really get a lot of feedback per se about the stuff we were recording. So there wasn’t, ‘Oh yeah, this is totally awesome.’ But we were liking it.”

Yet when a friend brought an advanced copy of the finished “Nevermind” to Channing, the drummer said he heard potential he hadn’t noticed before.

“I remember thinking when I got done listening to it, ‘God, this is a great record. I’ll bet this is gonna do pretty well,’” he said. “And, of course, later it got so huge it was like ‘Whoa, I didn’t think it’d get this big.’”

‘I’m so lonely, and that’s OK’
Brandon Geist, the editor of the rock and heavy metal magazine Revolver, was 13 years old when “Nevermind” came out. He said the main reason it had an impact on him and his generation was the seriousness of its lyrics.

“I remember hearing ‘Come As You Are’ on the radio,” Geist said. “It was one of those magic moments where it was like ‘What is this? This speaks to me in a way that nothing I’d heard before had.’”

Geist said that before “Nevermind,” commercial hard rock was “a very sort of macho genre,” where glam rock bands would sing about partying and women, thrash metal bands sang of political and social issues, and straight-up heavy metal bands sang about “Dungeons and Dragons” themes.

“But after ‘Nevermind’ hit, suddenly it was cool to be in a hard rock band and to sing about your feelings — and to sing about your feelings in a complex way,” Geist said. “Hard rock became inward-looking. You can see that influence in the nu metal bands like Korn or Slipknot. All of a sudden it was acceptable to be in a metal band and to sing about your neighbor molesting you or something. Hard rock really became cathartic as opposed to escapist.”

Inward-looking lyrical couplets like “I’m so lonely, and that’s OK” (from “Lithium”) also resonated with Nirvana’s musical peers — to the point where the focus of hard rock music was permanently changed — for better or worse, according to Eddy.

“Once Nirvana hit, there was this assumption that you had to present yourself seriously,” Eddy said. “So this kind of dreariness set into commercial hard rock that has not abated in the 20 years since, all the way up to the post-grunge bands Staind and Nickelback.”

Regardless of any detrimental influence Nirvana may have had on artists that came afterwards, new generations of listeners have been able to connect with the band’s music in much the same way the original grunge fans did back in 1991, Cross believes.

“What people forget is that Nirvana’s music reaches across generations and across cultures,” Cross said. “The songs that Kurt wrote were so deep and meaningful that they have continued to speak to new generations of people who find meaning in them.

“That, I think, is the reason for ‘Nevermind’s’ lasting legacy,” Cross said. “It’s not Kurt Cobain’s personality, it’s not who he was as a rock star. It’s simply that the songs themselves have a life beyond Kurt’s fame.”

Michael Azerrad, author of the 1993 best-seller “Come as You Are: The Story of Nirvana,” said by email that Nirvana’s music was able to resonate with teenagers because it spoke to them in an honest way.

“After years of inane dance-pop (Milli Vanilli) and inane hair farmer bands (Warrant), kids wanted rock music that spoke to and about them,” Azerrad wrote, “instead of focus-grouped junk approved by a bunch of greying, coked-up baby boomer record execs. Nirvana’s music had a crucial edge because it encapsulated a lot of the underground and alternative music those kids probably heard bits of but hadn't been able to get into because it was too raw and unmelodic. And it didn't hurt that the singer was kind of cute.”

“Nevermind” still holds an important place in the rock pantheon because it was released at a time when pop music hadn’t become so divided into subgenres and a hit rock album could resonate far beyond its core audience, Cross said. Consequently, it’s looked back upon as perhaps the final game-changing rock album.

“In that era there was such a thing as a rock artist that cut across all demographics and spectrums,” Cross said. “Now you don’t even have heavy metal anymore; you have death metal and you have speed metal and all the other fractionalization. What has happened in the last two decades is that music has become so specific that there are no longer artists of any sort that have that mainstream crossover power.”

Here we are now, entertain us ...

The members of Nirvana, from left, singer/guitarist Kurt Cobain, bassist Krist Novoselic and drummer Dave Grohl are photographed backstage at England's Reading Festival on Aug. 23, 1991. Originally from Aberdeen, Wash., the band was on the cusp of a worldwide explosion in popularity as part of the grunge explosion out of Seattle.
(Everett Collection)
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The almighty dollar

Nirvana released "Nevermind," its second studio album, on Sept. 24, 1991. Driven by the popularity of the first single, "Smells Like Teen Spirit," the hard-driving album went on to sell over 30 million copies worldwide.

Spencer Elden, the baby who appeared on the "Nevermind" cover, told MTV as a 17-year-old in 2008, "It's kind of creepy [to think] that that many people have seen me naked."
(DGC Records)
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Give him a lift

Cobain surfs the crowd during a show in Frankfurt, Germany, in December 1991. "Nevermind" had entered the Billboard Top 40 and "Smells Like Teen Spirit" was a genuine hit as the band toured Europe.
(Paul Bergen via Getty Images)
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Mainstream mindset

As the grunge wave rippled across the music industry and the cultural landscape, Nirvana appeared on the April 1992 cover of Rolling Stone magazine. Cobain sent his own message about selling out with a T-shirt reading "Corporate magazines still suck."
(Rolling Stone)
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An Axl to grind

Cobain performs on stage with Nirvana at the MTV Video Music Awards on Sept. 10, 1992. MTV wanted the band to play "Smells Like Teen Spirt," but Nirvana wanted to play a newer song -- "Rape Me." The two sides agreed that the band would play "Lithium" but Cobain opened the performance with the first few lines of "Rape Me" before changing gears.

Also at the event, Cobain got into it with Guns N' Roses frontman Axl Rose, who challenged the Nirvana singer to a fight backstage after exhchanging words with Cobain's wife Courtney Love.
(Frank Micelotta / Getty Images)
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Where did you sleep last night?

Nirvana taped an episode of MTV's "Unplugged" series at Sony Studios in New York on Nov. 18, 1993. The band's set included six covers, including David Bowie's "The Man Who Sold the World," as well as an appearance by the Meat Puppets for three songs.

Cobain died on April 5, 1994, of a self-inflicted shotgun wound. He was 27.

The album for the session was released in the wake of Cobain's suicide and debuted at No. 1 on the Billboard 200.
(Frank Micelotta / Getty Images)
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Always loud

Cobain and Nirvana perform during the taping of MTV's "Live and Loud" production in Seattle on Dec. 13, 1993.

The band had by then released its third studio album, "In Utero," and embarked on its first American tour in two years.

Rolling Stone reviewer David Fricke gave the album four-and-a-half out of five stars and wrote, "In Utero is a lot of things -- brilliant, corrosive, enraged and thoughtful, most of them all at once. But more than anything, it's a triumph of the will."
(Robert Sorbo / AP)
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A band of his own

Former Nirvana drummer Dave Grohl, performing at the MTV Movie Awards on June 5, 2011, has gone on to find success of his own as the frontman for the band Foo Fighters. The band has released seven studio albums since forming in 1994.
(Matt Sayles / AP file)
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Bucking the trends

Novoselic, left, and Grohl, center, pose with Butch Vig, the producer of the Nirvana's "Nevermind," in Los Angeles on Sept. 6, 2011.

After 20 years, "The album holds up really well. It's timeless," Vig told The Associated Press. "I think part of that is because the songs are really good. The production isn't gimmicky. It's just bass, drums and guitar. There's not any sort of trendy sound."
(Chris Pizzello / AP)
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Pieces of rock history

Nirvana artifacts are seen at the opening of "In Bloom: The Nirvana Exhibition," marking the 20th anniversary of the release of "Nevermind," at the Loading Bay Gallery in London on Sept. 13, 2011.
(Samir Hussein / Getty Images)
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